Silicon photonics receive phased array sensors

ABSTRACT

High-performance ultra-wideband Phased Array Sensors (PAS) are disclosed, which have unique capabilities, enabled through photonic integrated circuits and novel optical architectures. Unique capabilities for a Receive PAS are provided by wafer scale photonic integration including heterogeneous integration of III-V materials and ultra-low-loss silicon nitride waveguides, combining key component technologies into complex PIC devices. Novel aspects include optical multiplexing combining wavelength division multiplexing and/or a novel extension to array photodetectors providing the capability to combine many RF photonic signals with very low loss. The architecture also includes optical down-conversion, as well as digital signal processing to improve the linearity of the system. Simultaneous multi-channel beamforming is achieved through optical power splitting of optical signals to create multiple exact replicas of the signals that are then processed independently.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of the U.S. patent applicationSer. No. 16/156,087 “SILICON PHOTONICS RECEIVE PHASED ARRAY SENSORS”filed on Oct. 10, 2018, which is a continuation of the U.S. patentapplication Ser. No. 15/399,563 “SILICON PHOTONICS RECEIVE PHASED ARRAYSENSORS” filed on Jan. 5, 2017, which will be issued as a patent10234701 on Mar. 19, 2019, which claims priority to U.S. provisionalpatent application No. 62/274,904 “HETEROGENEOUSLY INTEGRATED WAFERSCALE SILICON PHOTONICS FOR RECEIVE PHASED ARRAY SENSORS” filed on Jan.5, 2016, all of them fully incorporated herein by reference.

STATEMENT REGARDING FEDERAL SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with U.S. Government support under ContractFA8650-15-C-1863, an Air Force SBIR Project, and the U.S. Government hascertain rights in the invention.

FIELD OF INVENTION

This invention relates to high performance active radio frequency (RF)phased array antenna systems, RF beamforming systems, RF down-converterand channelizer systems, and RF photonics systems, enabled by photonicintegrated circuit (PIC) devices.

BACKGROUND

CMOS foundry based silicon photonics devices provide the opportunity forhigh volume, low cost devices; ideal candidates for use in Phased ArraySensor (PAS) systems, which require processing for 1000's of elementsper antenna, supporting the use of large array Photonic IntegratedCircuit (PIC) devices. Advances in photonic integration, includingheterogeneous integration of III-V materials to fabricate lasers, linearmodulators, and photodetectors; ultra-low-loss silicon nitridewaveguides for high performance filters and time delay devices; andheterogeneous integration of magneto optic materials to create opticalisolators; enable the design and fabrication of complex PIC devices thatcan provide unique capabilities for PAS systems that cannot be obtainedwith electronic technologies. This invention utilizes these advancedphotonic devices integrated onto large size (wafer scale) PIC devices toenable a future generation of advanced PAS systems.

SUMMARY

This section highlights some of the key requirements for future Receive(Rx)-PAS systems, and how these can be achieved using PIC technology andnovel photonic architectures. Advanced PIC based Rx-PAS systems canprovide unique capabilities, including;

1. Ultra-Wideband Operation, with;

2. Multiple Channel Simultaneous RF Beamforming

3. Optical Gain

4. Optical Down-Conversion

5. True Time Delay

6. Tunable Optical Rejection of Interfering Signals

7. Optically Generated Ultra-Wideband Local Oscillator

8. Channelizer functionality

Ultra-Wideband Operation over very large frequency ranges, e.g. 1-40GHz, as well as large instantaneous bandwidth, 1 GHz and higher, whilealso providing the following unique capabilities;

Multiple Channel Simultaneous RF Beamforming is the equivalent of havingmultiple separate complete electronic Rx-PAS systems all housed within asingle Rx-PAS. This is achieved in this novel architecture throughoptical power splitting of the RF and Local Oscillator (LO) opticalsignals to create multiple exact replicas of the signals that are thenprocessed independently within the PIC device.

Optical Gain in the Rx-PAS architecture provides scaling of opticalperformance with the number of antenna elements, taking advantage of thelarge number of antenna elements for high optical gain. This gain canovercome the initial conversion loss from electronic to photonicsignals, support the optical splitting required for multiple channelsimultaneous RF beamforming, and support high spurious free dynamicrange (SFDR) system operation.

Optical Down-Conversion is a photonic capability that converts a chosenfrequency band down to the IF band by simply replacing the opticalcarrier with an optical LO carrier at the appropriate frequency. Thisovercomes the large mixing spurs that occur using electronic mixers thatsignificantly degrade SFDR in wide instantaneous bandwidth systems.

True Time Delay devices with large tunable delay, wide bandwidth andhigh system SFDR based on ultra-low loss silicon nitride waveguides andmicroresonators.

Tunable Optical Rejection of Interfering Signals across the fullbandwidth of future PAS systems can be provided by ultra-high-Q opticalmicroresonator filters designed in silicon nitride waveguides. Thiscapability can be used to eliminate co-site interference or jammingsignals from an adversary.

An Optically Generated UWB Local Oscillator can be added throughintegration of two frequency locked ultra-low phase noise lasers. Anoptically generated LO eliminates the need for a low phase noise RF LOsignal at every antenna element, and provides a much wider LO frequencyrange, e.g. 1-100 GHz.

Channelizer functionality is an inherent capability of the tunableoptical down-conversion Rx-PAS, which provides a tunable singlefrequency band channelizer. Using a novel system architecture this canbe expanded to create a tunable multiple frequency band channelizer, orin the limit a full channelizer.

Receive Phased Array Sensor (Rx-PAS) Optical Architectures

The novel system optical architectures of this invention provide a PICenabled Rx-PAS that can provide performance required in a future system,e.g. 8 or more simultaneous RF beams with ultra-wide instantaneousbandwidth (≥1 GHz), a wide operating frequency range (1-40 GHz), highSFDR (≥120 dB·Hz^(2/3)) and low Noise Figure (NF), <10 dB. The Rx-PASdesign was validated through system simulations, which are described inthe detailed description.

The Multiple Channel Simultaneous Beamforming (MCSB)-PIC device formsthe basis of the optical architecture. This MCSB-PIC provides completeoptical processing of signals from, e.g. 64 antenna elements in an 8×8array, including; converting the antenna electrical signal to an RFphotonic signal, filtering, time delaying and attenuating that RFsignal, generating an LO optical signal—also with appropriate opticalfiltering (if required), phase control and attenuation, down-convertingthe required frequency band of the RF signal to baseband (opticaldown-conversion) by combining the RF and LO signals, then combining allsignals to provide a single output for the 64 antenna elements. Multiplesimultaneous beamforming is achieved by optically splitting the RFPhotonic and LO signals from each antenna element, e.g. a simple 1:8optical power splitter, and then each of these channels is processedseparately by passing through their own set of filters, TTD,attenuators, combining, etc. The use of large scale integration throughthe silicon photonics platform, utilizing CMOS compatible designs and aCMOS foundry, allows the signal paths to be replicated many times onchip, e.g. 64×, to provide the required functionality on a single PICdevice.

A generic Rx-PAS system is shown in FIGS. 1 and 2; the front side of theantenna in FIG. 1 includes 1024 antenna elements (32×32), while thebackside of that antenna in FIG. 2 includes 16 wafer scale MCSB-PICmodules tiled across the antenna, each MCSB-PIC supporting 64 antennaelements (8×8). Large size (wafer scale) devices can provide all therequired functionality for 64 antenna elements within a single PIC.

Two optical architectures are proposed;

Full Optical System; including two-stage optical multiplexing, e.g.1024× or 4096×, as shown in FIGS. 3 and 4. In the full optical system,the wavelength division multiplexed signals for each simultaneouschannel exit each MCSB-PIC device for a second stage of opticalmultiplexing in either an array photodetector, or in the proposed novelGroup Array Photodetector Combiner (GAPC) device. This two stagemultiplexing provides tremendous optical gain, e.g. 1024 optical signalsare combined to provide a single RF output, which provides for highsystem performance.

Modular Hybrid Optical/Digital (MHOD) System; single stage multiplexing,e.g. 64×, as shown in FIG. 5. The MHOD system, which provides all theunique capabilities of the optical approach, with e.g. 64× opticalsignal multiplexing (less optical gain than the Full Optical System),with the electrical output for each simultaneous channel of eachMCSB-PIC device being digitized and processed with a digital signalprocessor (DSP) before the 16 digital signals for each simultaneouschannel are combined to form an Rx-PAS RF output. The DSP can also beutilized to increase the linearity of the system, improving SFDR, byremoving the non-linear (sinusoidal) transfer characteristic of themodulators and other nonlinear elements in the optical signal path; thiscan be accomplished by applying the inverse transfer response in theDSP, i.e. digital post-processing equalization. A similar DSP can beutilized in the full optical system to increase SFDR, however, in thatcase only one DSP is required per simultaneous beamforming channel.

The MHOD design allows for all optical components and interconnects tobe within a single PIC device with no optical inputs or outputs; onlyelectrical connections to the antenna, LO, RF outputs, and controlsignals are required.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. Example schematic for a Receive Phased Array Sensor (Rx-PAS)showing the antenna side with 32×32 (1024 total) antenna elements

FIG. 2. Example schematic for a Rx-PAS showing wafer scaled MultipleChannel Simultaneous Beamforming-Photonic Integrated Circuit (MCSB-PIC)devices (modules) tiled across the back of the antenna; electricallyconnected to all antenna elements, each MCSB-PIC supporting 8×8 antennaelements (16 MCSB-PICs total).

FIG. 3. Rx-PAS system utilizing two stage optical multiplexing;wavelength division multiplexing (WDM) on the first stage followed by aGrouped Array Photodetector Combiner (GAPC) for the second stage.

FIG. 4. Schematic of two stage optical multiplexing system, using WDM tocombine N optical signals from N antenna elements (for each MCSB-PIC),followed by a GAPC to combine the M multiplexed signals from M separateMCSB-PICs.

FIG. 5. Modular Hybrid Optical/Digital (MHOD) system schematic; opticalbeamforming on a single MCSB-PIC (e.g. supporting 8×8 antenna elements),with an analog to digital converter (ADC) and digital signal processing(DSP) per MCSB-PIC beamforming channel, followed by digital beamformingto combine information for all the elements of the array (additionaldigital beamforming elements not shown).

FIG. 6. Basic Rx-PAS system design schematic. One laser and signalchannel per antenna element; the laser carrier is modulated by theelectrical signal from the antenna element; this signal is time delayedand amplitude controlled; the optical signals from all antenna elementsare combined to provide the beamformed output.

FIG. 7. Shows the use of an Optical Splitter in the Basic Rx-PAS systemdesign to provide multiple simultaneous beamforming, a single opticalchannel is shown; the optical power splitter provides a set of Xidentical optical signals (8 shown), which can be independentlyprocessed (time delayed, amplitude controlled, and combined) to providebeamforming of X simultaneous and independent beams.

FIG. 8. Rx-PAS system including optical down-conversion to provide theoutput signal near baseband to utilize high performance ADCs; thisdesign uses 2 locked lasers per channel, one for the antenna opticalsignal and one for the optical Local Oscillator (LO) to be used fordown-conversion.

FIG. 9. Shows the use of an Optical Splitter in the Rx-PAS systemincluding optical down-conversion using 2 locked lasers, to providemultiple simultaneous beams (8 shown); a single optical channel isshown.

FIG. 10. Rx-PAS system with optical down-conversion using 1 laser whichis power split to provide both the antenna signal and LO signal. The LOoptical signal is obtained by modulating the laser carrier with an LO RFsignal, then filtering off the required optical LO signal.

FIG. 11. MCSB-PIC design using wavelength division multiplexing (WDM)for signal combining. The optical multiplexed signal can leave theMCSB-PIC for 2 stage multiplexing, or a photodetector (or differentialpair of photodetectors) on the MCSB-PIC can convert the multiplexedoptical signal to a single electrical signal.

FIG. 12. MCSB-PIC using a single GAPC device for signal combining,providing a single electrical output.

FIG. 13. Simulated spurious free dynamic range (SFDR) versus the numberof lasers multiplexed together in the Rx-PAS system (1 or 2 lasers perantenna element). These simulations compare the Rx-PAS system SFDR usingeither a standard Mach Zehnder interferometer (MZI) modulator or aLinearized MZI modulator, and also using a laser with standard relativeintensity noise (RIN) of −150 dB/Hz, or low RIN of −170 dB/Hz. A goalfor SFDR of 120 dB·Hz²¹³ is included in the plot.

FIG. 14. Simulated noise figure (NF) versus the number of lasersmultiplexed together in the Rx-PAS system (1 or 2 lasers per antennaelement). The NF is shown for the Rx-PAS system shown in FIG. 3, thesystem without a semiconductor optical amplifier (SOA), and also for adesign including an SOA following each antenna modulator in order toincrease the photonic signal level.

FIG. 15. Schematic of a 64 photodetector GAPC made from 16 Groups of 4photodetectors. The equivalent circuit for the GAPC includes acapacitance for the Group of photodetectors, C, (plus internalresistance), plus the inductors L and L/2 chosen to create an artificialtransmission line.

FIG. 16. Schematic of a 64 photodetector GAPC made from 8 Groups of 8photodetectors. The equivalent circuit for the GAPC includes acapacitance for the Group of photodetectors, C, (plus internalresistance), plus the inductors L and L/2 chosen to create an artificialtransmission line.

FIG. 17. Shows the extension from a single tunable down-convertedchannel, which can be used as a tunable single frequency bandchannelizer, to a multiple tunable frequency band channelizer (in thelimit all frequency bands can be down-converted making a full coveragechannelizer). In this schematic, only a single optical channel (of Nchannels) is shown.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of the Rx-PAS shown in FIGS. 1 and 2 is an example thatuses 16 MCSB-PIC modules tiled across the back of an antenna, eachMCSB-PIC supporting 64 antenna elements (8×8), with all 16 devicessupporting the 1024 element (32×32) PAA. The large size (wafer scale)devices can provide all the required functionality for 64 antennaelements within a single PIC. RF connections are made through theantenna from each antenna element to the appropriate modulator on eachMCSB-PIC.

FIG. 3 shows the Full Optical System, which includes wavelengthmultiplexing within each MCSB-PIC device to provide a single opticaloutput per beamforming channel, the outputs from all MCSB-PIC devicesfor each channel being combined in the second stage of opticalmultiplexing in an array photodetector, such as the proposed GAPCdevice. WDM on the MCSB-PIC allows the combination of 64 signals,separated in wavelength to avoid noise due to overlapping channelsinterfering with each other. The array multiplexing for the second stagekeeps the WDM signals from each MCSB-PIC on separate photodetectorelements, again to eliminate possible interference effects.

A schematic of the 2 stage multiplexing scheme 10 is shown in FIG. 4.Within the first MCSB-PIC, 20, signals from antenna elements 21 and 23drive the associated wavelength specific channels 22 and 24, the outputsfrom all channels on the MCSB-PIC 20 being combined in a wavelengthdivision multiplexer 25. Multiple MCSB-PICs form the Rx-PAS system. Thefinal, M^(th) MCSB-PIC, 30, uses signals from antenna elements 31 and 33to drive the associated wavelength specific channels 32 and 34, theoutputs from all channels on this MCSB-PIC 30 being combined in awavelength division multiplexer 35. Each MCSB-PIC has one optical outputper simultaneous beamforming channel, although only one channel is shownin FIG. 4. The outputs from all MCSB-PIC devices for a specificsimultaneous beamforming channel are combined within the GAPC device 40,providing a single RF output per simultaneous beamforming channel 50.The GAPC device combines outputs from M MCSB-PIC devices.

The MHOD system design is shown schematically in FIG. 5. Instead oftwo-stage optical multiplexing, in which optical signals must leave eachMCSB-PIC to go to the GAPC device, within the MHOD design all opticalsignals stay on the MCSB-PIC, with only electrical input and outputsignals. In this case, the MCSB-PIC can include a wavelength divisionmultiplexer to combine all the optical signals (which are at differentWDM wavelengths), followed by a photodetector, or a differential (alsoknown as balanced) pair of photodetectors; or the MCSB-PIC can include aGAPC device to combine the many optical signals without concern fortheir wavelengths, i.e. the channels do not have to be at specificwavelengths. The GAPC device can again be formed of a single array, or adifferential pair array.

FIG. 6 shows an embodiment of the basic Rx-PAS system design 100. Thesystem is made up of N channels, each channel supporting an antennaelement. Channel 1, 110, includes a signal laser 111, an antenna element112 which drives a modulator 113 and modulates the carrier from thelaser 111. The modulated optical carrier passes through a tunable timedelay device 114, which imparts a chosen time delay on the modulatedsignal, followed by an amplitude control device, or attenuator 115,which controls the size of the signal. The N^(th) channel, 120, includessignal laser 121, antenna element 122, modulator 123, tunable time delay124, and amplitude control/attenuator 125. Optical signals from all Nchannels, providing delayed and attenuated signals associated with Nantenna elements, are combined in combiner device 130, providing asingle RF beam-formed output 140. The beamforming is chosen by thevalues of time delay and attenuation for each channel. FIG. 6 shows thebasic Rx-PAS system design for a single beamforming channel. Thebeam-formed RF output of this and other Rx-PAS systems in this inventionare typically electrically band-pass filtered, and then passed into ananalog to digital converter (ADC), followed by a DSP that is used toprocess information for that beamforming channel. Digital processing canbe utilized to further linearize the system performance, increasingSFDR, counteracting the nonlinearity of components in the systems byapplying the inverse characteristic, e.g. the sinusoidal transfercharacteristic of the MZI modulator used in most systems can belinearized by providing the inverse transfer characteristic of themodulator.

FIG. 7 shows how an optical power splitter can be used to createmultiple simultaneous beams. FIG. 7 shows a single channel of the basicRx-PAS system design shown in FIG. 6, expanded through the use of anoptical power splitter, to create multiple output beams, in this example8 beams. The power splitting scheme for channel 1, 200, includes asignal laser 201, antenna element 202, and modulator 203, which togetherprovide a single modulated optical carrier based on the electricalsignal from the antenna element. The modulated optical carrier is splitinto eight identical optical signals, each the same as the modulatedoptical carrier but smaller in size, by passing through an optical powersplitter 204. Each of these identical optical signals passes through itsown time delay device, attenuator, and then goes to the appropriatecombiner for the specific output beam. The first of eight outputs fromthe splitter passes through time delay 211, amplitude control/attenuator212 and on to the combiner for beam 1, 210. The last of eight outputsfrom the splitter passes through time delay 221, amplitudecontrol/attenuator 222 and on to the combiner for beam 8, 220. Followingthe optical power splitter, all elements for each specific beam are thesame as would be required for a single beam. The use of the siliconphotonics integration platform, using CMOS foundry processing, enablesthe large scale integration required for this multiple simultaneousbeamforming system.

FIG. 8 shows an extension of the basic Rx-PAS system design 100 toinclude optical down-conversion within the system. The Down-convertingRx-PAS system 300, includes N channels, supporting N antenna elements,with the outputs from all N channels combined to provide the singlebeamforming RF output 340. FIG. 8 shows the down-converting Rx-PASsystem for a single beamforming channel. The signal laser 310, forchannel 1, has the electrical signal from antenna element 311 modulatedonto the laser carrier by modulator 312. The frequency band that isrequired to be down-converted, is selected from the modulated signalusing filter device 313; this chosen frequency band signal is then timedelayed in time delay device 314, attenuated in amplitude control device315, and then passes into coupler 319. A separate LO laser is locked tothe signal laser with a specific offset frequency; this can beaccomplished by frequency locking both lasers to a single ultra-high Qreference filter, each laser locked to a different resonance, with thechosen offset frequency set by the chosen resonances. For channel 1, theoffset frequency of the LO laser 316 is chosen to align with thefrequency band selected by filter 313, to enable down-conversion. The LOlaser output is phase delayed in phase delay device 317, amplitudecontrolled in attenuator device 318, and then passes into coupler 319.The coupler 319, combines the channel 1 antenna signal and channel 1 LOsignal, which replaces the optical carrier of the signal laser (whichwas removed by filter 313) with the LO signal, effectively creating adown-converted optical signal. The coupler 319, provides two outputswhich are out of phase with one another; these are used withdifferential detection to cancel the relative intensity noise (RIN)effects of the laser and even order distortion products. Each of the Nchannels support a different antenna element, with the same series ofoptical elements. The N^(th) channel has a signal laser 320 and LO laser326, frequency locked with the same offset frequency as channel 1, andall the channels within 300. Antenna element 321 provides the electricalsignal for modulator 322, modulating the carrier of the signal laser320. Filter 323 selects the same frequency band from the modulatedsignal as filter 313 selected in channel 1. This chosen frequency bandsignal is then time delayed in time delay device 324, attenuated inamplitude control device 325, and then passes into coupler 329. The LOlaser 326 output is phase delayed in phase delay device 327, amplitudecontrolled in attenuator device 328, and then passes into coupler 329.The coupler 329, combines the channel N antenna signal and channel N LOsignal, which replaces the optical carrier of the signal laser (whichwas removed by filter 323) with the LO signal, effectively creating adown-converted optical signal. The outputs from all N couplers, for allN channels, are combined in the combiner 330, providing a single RFbeamforming output 340. The combiner 330 includes a differential pair ofcombiners, one for each of the two outputs of each coupler.

FIG. 9 shows how optical power splitters can be used to create multiplesimultaneous beams for the Down-converting Rx-PAS system shown in FIG.8. Similar to the power splitting used in the Standard Rx-PAS system(FIG. 7), optical power splitters are used to create identical copies (8copies in FIG. 9) of both antenna optical signals and LO signals, withthese signals then passing through the same parallel set of photoniccomponents to create individual, simultaneous down-converted RF beams.FIG. 9 shows optical power splitting and simultaneous beamforming (8simultaneous beams) for channel 1 only, supporting a single antennaelement; in the full system this is replicated for all N antennachannels. The carrier of the signal laser 410 is modulated by theelectrical signal from antenna element 411, using modulator 412, andthis modulated optical signal is split into 8 identical modulatedoptical signals, each the same as the modulated optical carrier butsmaller in size, by passing through an optical power splitter 413. Eachof these identical optical signals passes through its own time delaydevice, attenuator, and then goes to the appropriate coupler for thespecific output beam. The first of eight outputs from the splitter 413passes through time delay 431, amplitude control/attenuator 432 and onto the coupler for beam 1, 430. The last of eight outputs from thesplitter passes through time delay 441, amplitude control/attenuator 442and on to the coupler for beam 8, 440. Similarly, the output of theoffset frequency locked LO laser 420 is split into 8 identical opticalsignals, each the same as the LO optical carrier but smaller in size, bypassing through an optical power splitter 421. Each of these identicaloptical signals passes through its own phase delay device, attenuator,and then goes to the appropriate coupler for the specific output beam.The first of eight outputs from the splitter 421 passes through phasedelay 451, amplitude control/attenuator 452 and on to the coupler forbeam 1, 450. The last of eight outputs from the splitter passes throughphase delay 461, amplitude control/attenuator 462 and on to the couplerfor beam 8, 460. For the complete system, the couplers from all Nchannels, for a specific beam (e.g. beam 1), all go into a combiner toprovide RF beam output 1. Similarly, combiners for each of the 8 beamsprovide 8 RF outputs.

FIG. 10 shows an alternative design for the Down-converting Rx-PASsystem 500, which replaces the two separate locked lasers in FIG. 8 witha single laser that is split to provide the two optical carriers thatform the antenna modulated signal and the LO optical signal. Channel 1includes a single laser 510, the output of which is split in two, withthe splitting ratio optimized for best system performance. Part of thelaser carrier is modulated by modulator 512, driven by the electricalsignal from antenna element 511. The frequency band that is required tobe down-converted, is selected from the modulated signal using filterdevice 513; this chosen frequency band signal is then time delayed intime delay device 514, attenuated in amplitude control device 515, andthen passes into coupler 521. The other part of the laser carrier ismodulated in modulator 517, using the LO RF signal 516, and then thissignal is filtered to select the required LO optical signal by filter518. The filtered LO optical signal is then phase delayed in phase delaydevice 519, attenuated in amplitude control device 520, and then passesinto coupler 521. Similarly for channel N, the output of laser 530 issplit in two, with the splitting ratio optimized for best systemperformance. Part of the laser carrier is modulated by modulator 532,driven by the electrical signal from antenna element 531. The frequencyband that is required to be down-converted, is selected from themodulated signal using filter device 533; this chosen frequency bandsignal is then time delayed in time delay device 534, attenuated inamplitude control device 535, and then passes into coupler 541. Theother part of the laser carrier is modulated in modulator 537, using theLO RF signal 536, and then this signal is filtered to select therequired LO optical signal by filter 538. The filtered LO optical signalis then phase delayed in phase delay device 539, attenuated in amplitudecontrol device 540, and then passes into coupler 541. Coupler outputsfrom all N channels are combined within the combiner 550, providing asingle RF beamforming RF output 560. FIG. 10 shows the schematic for asingle beamforming channel, as in FIG. 8. This can be extended to createmultiple beamforming using optical splitters, similar to that shown inFIG. 9.

FIG. 11 shows a similar Down-converting Rx-PAS system design to that inFIG. 10, however, in this case, the N channels utilize N lasers withdifferent and separate wavelengths, and use a wavelength multiplexer inthe combiner element, i.e. taking advantage of WDM. Channel 1 includes asingle wavelength specific laser (WDM laser) 610, the output of which issplit in two, with the splitting ratio optimized for best systemperformance. Part of the laser carrier is modulated by modulator 612,driven by the electrical signal from antenna element 611. The frequencyband that is required to be down-converted, is selected from themodulated signal using filter device 613; this chosen frequency bandsignal is then time delayed in time delay device 614, attenuated inamplitude control device 615, and then passes into coupler 621. Theother part of the laser carrier is modulated in modulator 617, using theLO RF signal 616, and then this signal is filtered to select therequired LO optical signal by filter 618. The filtered LO optical signalis then phase delayed in phase delay device 619, attenuated in amplitudecontrol device 620, and then passes into coupler 621. Similarly forchannel N, the output of laser 630 is split in two, with the splittingratio optimized for best system performance. Part of the laser carrieris modulated by modulator 632, driven by the electrical signal fromantenna element 631. The frequency band that is required to bedown-converted, is selected from the modulated signal using filterdevice 633; this chosen frequency band signal is then time delayed intime delay device 634, attenuated in amplitude control device 635, andthen passes into coupler 641. The other part of the laser carrier ismodulated in modulator 637, using the LO RF signal 636, and then thissignal is filtered to select the required LO optical signal by filter638. The filtered LO optical signal is then phase delayed in phase delaydevice 639, attenuated in amplitude control device 640, and then passesinto coupler 641. Coupler outputs from all N channels are combinedwithin the combiner 650, providing a single RF beamforming RF output660. FIG. 11 shows the schematic for a single beamforming channel, as inFIG. 8. This can be extended to create multiple beamforming RF outputsusing optical power splitters, similar to that shown in FIG. 9.

FIG. 12 shows a similar Down-converting Rx-PAS system design to that inFIG. 10, however, in this case, the combiner element is a GAPC device.Channel 1 includes a single non-wavelength specific laser (i.e. specificwavelength is not important, and could be the same, similar, ordifferent for the different channels) 710, the output of which is splitin two, with the splitting ratio optimized for best system performance.Part of the laser carrier is modulated by modulator 712, driven by theelectrical signal from antenna element 711. The frequency band that isrequired to be down-converted, is selected from the modulated signalusing filter device 713; this chosen frequency band signal is then timedelayed in time delay device 714, attenuated in amplitude control device715, and then passes into coupler 721. The other part of the lasercarrier is modulated in modulator 717, using the LO RF signal 716, andthen this signal is filtered to select the required LO optical signal byfilter 718. The filtered LO optical signal is then phase delayed inphase delay device 719, attenuated in amplitude control device 720, andthen passes into coupler 721. Similarly for channel N, the output of thenon-wavelength specific laser 730 is split in two, with the splittingratio optimized for best system performance. Part of the laser carrieris modulated by modulator 732, driven by the electrical signal fromantenna element 731. The frequency band that is required to bedown-converted, is selected from the modulated signal using filterdevice 733; this chosen frequency band signal is then time delayed intime delay device 734, attenuated in amplitude control device 735, andthen passes into coupler 741. The other part of the laser carrier ismodulated in modulator 737, using the LO RF signal 736, and then thissignal is filtered to select the required LO optical signal by filter738. The filtered LO optical signal is then phase delayed in phase delaydevice 739, attenuated in amplitude control device 740, and then passesinto coupler 741. Coupler outputs from all N channels are combinedwithin the GAPC device 750, providing a single RF beamforming RF output760. FIG. 12 shows the schematic for a single beamforming channel, as inFIG. 8. This can be extended to create multiple beamforming usingoptical power splitters, as shown in FIG. 9.

Examples of a simulated Down-converting Rx-PAS system performance, basedon the 2 stage multiplexing scheme shown in FIGS. 3 and 4, are shown inFIGS. 13 and 14. Calculated SFDR and Noise Figure (NF) are shown versusthe total number of lasers in a system (e.g. 64 for a single MCSB-PICsystem with 1 laser per channel, and 128 for a single MCSB-PIC systemwith 2 lasers per channel; larger numbers for 2 stage multiplexing). Thesimulated system includes a 1:8 optical power splitter in antenna and LOsignal arms to provide 8 independent, simultaneous beamforming RFoutputs. Results in FIG. 13 show that high SFDR can be reached using acombination of low RIN lasers (or RIN cancellation; which is includedthrough the use of differential photodetectors) and linearizedmodulators. Low NF is achieved with a larger number of lasers (highmultiplexing gain), or the addition of semiconductor optical amplifiers(SOAs) after the antenna modulators to increase the signal power for lownumbers of lasers, e.g. for a single MCSB-PIC device. Simulations werecarried out without including a low noise amplifier (LNA) at eachantenna element (these are a requirement in electronic PAS systems) toshow that the system can meet requirements without using LNAs, providinga lower power dissipation system and the potential for ultra-widebandwidth operation; which is otherwise limited by LNA bandwidth.

A novel extension of the standard travelling wave array photodetectorapproach is proposed as part of this invention. The Group ArrayPhotodetector Combiner (GAPC) device includes a large number ofphotodetectors e.g. 64 photodetectors, to combine the RF photonicsignals from 64 independent optical inputs, provided on 64 separatewaveguides. Within an MCSB-PIC this device can combine the signals fromall 64 of the PAS antenna elements. Within this novel approach, the 64photodetectors are split up into ‘Groups’ of a smaller number of PDs,e.g. 4 or 8, then these groups are combined within a synthetictransmission line structure to retain the bandwidth of the photodetectorgroups, to combine all outputs into a single electrical output. Thelarge number of photodetector elements is required to physicallyseparate the absorption volume of each channel, to avoid anyinterference effects that may occur if channel wavelengths overlap.Schematics for two such GAPC devices are shown in FIGS. 15 and 16. FIG.15 shows groups of 4 photodetectors, labeled Group 1 (G1) to G16. Forindividual photodetectors, which have high responsivity, e.g. ˜100%, andhigh speed, e.g. 38 GHz into 50 Ohms, combining a group of 4 devicesreduces the ˜3 dB bandwidth, to somewhat more than ¼ of 38 GHz becauseonly 1 bond pad (capacitance) is used for 4 photodetectors. Similarly,for a Group of 8 photodetectors, the −3 dB bandwidth is still wellbeyond that required for the Down-converting Rx-PAS. For the StandardRx-PAS system not including down-conversion, higher bandwidth GAPCdevices are required, which will utilize smaller groups of devices aswell as higher intrinsic bandwidth photodetectors.

The Groups are connected in the GAPC device with the correct inductancechosen to match the capacitance of the photodetector groups to create asynthetic 50 Ohm transmission line, i.e. Z=sqrt(L/C). With 50 Ohmmatching terminations at both ends of the device, the overall matchingresistance is 25Ω, increasing the −3 dB bandwidth of the array. For apractical 8×8 GAPC device, using an estimated capacitance from thephotodetector active area of 30 fF and that of the single bond pad of 40fF, the overall Group capacitance is 280 fF (values predicted for thephotodetectors in a planned fabrication run). While the capacitance isincreased by combining the photodetectors, reducing bandwidth, theseries resistance, e.g. 20 Ohms—a typical value for a high speedphotodetector, which typically limits the performance and number ofelements in a traveling wave photodetector due to RF power loss, issignificantly reduced (20/8=2.5Ω) by the parallel photodetectors. Thisgroup concept therefore significantly improves the traveling waveapproach through the reduced resistance (signal loss), and is useful aslong as the overall device bandwidth is sufficient for the application.Using these values for 8 Groups of 8 photodetectors, the bandwidth ofthe 64 GAPC device into 25Ω is found to be still high, i.e. 20.7 GHz.Additionally, the cutoff frequency for the transmission line iscalculated to be 22.7 GHz, using an inductance value, L, of 0.7 nH inthis case. This 8×8 GAPC design clearly has significantly more bandwidththan the ˜3 GHz flat bandwidth required for a Down-converting Rx-PASsystem, and sufficient for a Standard Rx-PAS system operating up to 20GHz.

Using this novel approach, a very large signal count, low loss, RFphotonic combiner can be fabricated, which can combine the 64 signals onthe MCSB-PIC, or more in future designs. One advantage of the GAPC isthat each photodetector has only one input optical signal, and so thewavelength of the 64 signals does not have to be controlled. Havingmultiple optical signals as input to one photodetector requiresdifferent wavelength signals to avoid coherent effects when the signalsare added; leading to the WDM approach option of the MCSB-PIC. Using the64 channel combiner therefore avoids the need for WDM lasers and a WDMmultiplexer in the single stage multiplexing version of the Rx-PAS, e.g.the MHOD system version. The spread of the optical power over manyelements (64) also improves the thermal performance of these devices,which is the ultimate limitation in many high power, high speedphotodetector applications.

The combiner or GAPC device used in Rx-PAS systems with two coupleroutputs, i.e. + and − outputs, will utilize differential photodetectorpairs for cancellation of RIN and even order distortion products; inthat case, the GAPC designs shown in FIGS. 15 and 16 will be doubled up,to provide one such device for the + coupler outputs and one for the −coupler outputs, the 2 GAPC devices being wired as a differential pair.

Considering the GAPC device 800 shown in FIG. 15, this device is made upof 16 groups of 4 photodetectors. The first group, G1, 810, which hascapacitance C, is made up of 4 high speed photodetectors, 815, 816, 817,and 818, with 4 input optical waveguides 811, 812, 813 and 814respectively. The 4 high speed photodetectors are electrically directlyconnected to each other, so that their photo-detected currents arecombined, and they share one bonding pad shown under the label G1. Theytherefore act like a single larger photodetector, but with 4 separate(independent) inputs. The device includes 16 groups, from G1 to G16. Thelast group, G16, is made up of 4 high speed photodetectors, 845, 846,847, and 848, with 4 input optical waveguides 841, 842, 843 and 844respectively. The synthetic transmission line is made up of the 16Groups with capacitance C (and series resistance), matched with 15inductors L between the groups, 861 between G1 and G2, and 862 betweenG15 and G16. At the ends of the synthetic transmission line are two loadresistors, R_(L) of typically 50 Ohms, 851 and 852, these load resistorsseparated from the end Groups by inductors of value L/2, i.e. 863between R_(L) and G1, and 864 between R_(L) and G16. Optical inputs onall (64) waveguide inputs are converted to electrical signals andcombined into a single RF output signal, which can be obtained acrosseither load resistor R_(L).

FIG. 16 shows GAPC device 900, which is made up of 8 groups of 8photodetectors. The first group, G1, 920, which has capacitance C, ismade up of 8 high speed photodetectors, 921, 922, 923, 924, 925, 926,927 and 928, with 8 input optical waveguides 911, 912, 913, 914, 915,916, 917 and 918 respectively. The 8 high speed photodetectors areelectrically directly connected to each other, so that theirphoto-detected currents are combined, and they share one bonding padshown under the label G1. They therefore act like a single largerphotodetector, but with 8 separate (independent) inputs. The deviceincludes 8 groups, from G1 to G8. The last group, G8, is made up of 8high speed photodetectors, 961, 962, 963, 964, 965, 966, 967 and 968,with 8 input optical waveguides 951, 952, 953, 954, 955, 956, 957 and958 respectively. The synthetic transmission line is made up of the 8Groups with capacitance C (and series resistance), matched with 7inductors L between the groups, 981 between G1 and G2, and 982 betweenG7 and G8. At the ends of the synthetic transmission line are two loadresistors, R_(L) of typically 50 Ohms, 971 and 972, these load resistorsseparated from the end Groups by inductors of value L/2, i.e. 983between R_(L) and G1, and 984 between R_(L) and G8. Optical inputs onall (64) waveguide inputs are converted to electrical signals andcombined into a single RF output signal, which can be obtained acrosseither load resistor R_(L).

This invention provides ultra-wideband, multiple simultaneousbeamforming over a wide frequency range, or alternatively it can providetunable optical down-conversion of a frequency band within the Rx-PASarchitecture to convert different RF frequency bands near baseband. Thistunable Down-conversion Rx-PAS is equivalent to a tunable, singlefrequency band RF channelizer, as a frequency band can be chosen andproduced at the beamforming RF output. The choice of beamformingparameters can provide a single tunable frequency band RF channelizerwith any beam-shape, from a broad beam to look in all directionstogether, to a narrow aimed channelizer beam-shape. The single tunablefrequency band RF channelizer can be expanded to create a multipletunable frequency band RF channelizer, as shown in FIG. 17, or in thelimit when all channels are included, this provides a full RFchannelizer. An advantage of using tunable channelizer frequency bandsis that fewer ADC and following electronics are necessary when allchannels are not populated; reducing overall component size, power andcost. FIG. 17 shows a multiple frequency band RF channelizer schematic,in this case using multiple locked LO lasers to provide the optical LO'sfor each frequency band (and eliminating the need for many tunable RF LOsignals)—extending the locking approach described in FIG. 8.Alternatively, the multiple LOs could be generated from a single lasersplit into multiple outputs and modulated with an RF LO signal, whichfor a larger number of LOs could include optical amplification (e.g.with an SOA) to increase the power level of each LO.

Within FIG. 17, channel 1 of a multiple frequency band RF channelizer isshown; 1000. The channel 1 antenna laser 1010 is modulated by themodulator 1030, using the electrical signal from antenna element 1020.The modulated carrier signal passes through a series of tunable filters,the first filter 1040 and the last filter, Filter Y 1050. Each in theseries of tunable filters selects a frequency band for down-conversion,and passes the remaining modulated signal along to the next filter inline. Filter 1, 1040, selects a frequency band which passes through timedelay device 1060, amplitude control device 1070, and into coupler 1,1130. The output from LO laser 1, 1100, passes through phase controldevice 1110, through amplitude control device 1120, and into coupler 1,1130. Coupler 1 provides + and − outputs for the channel 1 input to acombiner for frequency band 1. This combiner has inputs from + and −coupler outputs of all N channels, and provides a frequency band 1beam-formed RF output. Each filter and LO pair are combined in a couplerto provide + and − outputs to the combiner for the associated frequencyband. In FIG. 17, the last filter, Filter Y 1050, selects a frequencyband which passes through time delay device 1080, amplitude controldevice 1090, and into coupler 1, 1330. The output from LO laser Y, 1300,passes through phase control device 1310, through amplitude controldevice 1320, and into coupler 1, 1330. Coupler Y provides + and −outputs for the channel Y input to a combiner for frequency band Y. Thiscombiner has inputs from + and − coupler outputs of all N channels, andprovides a frequency band Y beam-formed RF output.

Although several exemplary embodiments have been herein shown anddescribed, those of skill in the art will recognize that manymodifications and variations are possible without departing from thespirit and scope of the invention, and it is intended to measure theinvention only by the appended claims.

What is claimed is: 1.-20. (canceled)
 21. A Group Array PhotodetectorCombiner (GAPC) device, comprising: an array of N photodetectors, whereN≥4, that receive a series of N optical signals, one optical signal toeach photodetector; wherein the array of N photodetectors forms a set ofX groups of photodetectors, where X≥2; wherein each group of the set ofX groups includes Y photodetectors directly electrically connected toeach other, where Y≥2 and where N=X*Y; wherein each group of the Xgroups of photodetectors is electrically connected to a next group ofthe X groups of photodetectors via an inductor, each said inductorhaving an inductance L; and wherein inductance values of each inductorare chosen to match a capacitance C of a corresponding group ofphotodetectors to produce an artificial transmission line having animpedance Z, using inductor values L within the artificial transmissionline and inductor values L/2 at ends of the artificial transmissionline, where Z=sqrt(L/C).
 22. The device of claim 21, wherein allphotodetectors are waveguide photodetectors having individual waveguideinputs.
 23. The device of claim 21, wherein an electrical output fromthe GAPC is the voltage across a load resistor at the end of theartificial transmission line.
 24. The device of claim 21, wherein eachoptical signal is comprised of multiple individual optical signals ofdifferent wavelengths, wherein there is an absence of any opticalwavelength overlap between each of the multiple individual opticalsignals.
 25. The device of claim 21, wherein each of the array of Nphotodetectors is physically separate so as to improve the thermalperformance and increase the overall device high-power capability. 26.The device of claim 25, wherein the optical signal input to eachphotodetector is comprised of multiple optical signals of differentwavelengths, wherein there is an absence of any wavelength overlapbetween each of the multiple optical signals.
 27. Two GAPC devicesaccording to claim 21, wherein each of said two GAPC devices arearranged as a differential pair through wiring.
 28. A phased arraysensor, comprising: an array of N antenna receiving elements, where N≥2,that receive a free space radio frequency (RF) signal; the RF signalbeing a mixture of RF signals coming from all directions; wherein eachof the receiving elements converts the received RF signal into anantenna element electrical signal; a combination of multiple photonicintegrated circuit (PICs), the combination of multiple PICs including: Noptical modulators configured to receive optical carriers from N lasers,the N modulators modulate the optical carriers with the antenna elementelectrical signal received from a corresponding one of the antennaelements; wherein all output optical signals from the N modulators passthrough N corresponding time delay (TD) elements, each of the TDelements imparts an individual time delay to each of the output opticalsignals from the N modulators, each TD element producing a TD elementoutput signal; and N amplitude controllers configured to individuallycontrol the TD element output signals from the N TD elements by applyingindividual attenuations; wherein, via a digital signal processor, all ofthe time delays and the attenuations are selected to collectivelyprovide from the free space RF signal only an RF signal with chosen beamforming parameters; and wherein output optical signals from all Namplitude controllers are combined in a combiner comprising a GAPC,wherein the electrical outputs from the group of photodetectors on eachPIC are connected to an adjacent group on an adjacent PIC via inductors,said GAPC outputting a single electrical output, the amplitude and phaseof which contain information about the RF signal with chosen beamforming parameters received by the N antenna receiving elements.
 29. Thesensor of claim 28, wherein the output signals from the N modulators areeach split into M identical copies of the output signals using anoptical power splitter, wherein M≥2, the M identical copies being usedto simultaneously provide M independent RF output signals, each RFoutput signal having independently chosen beam forming parameters byselecting appropriate time delays and attenuations.
 30. The sensor ofclaim 28, wherein the beam forming parameters include a directiondefined by polar and azimuthal receiving angles.
 31. The sensor of claim30, wherein the beam forming parameters include a selected angularspread of the received RF signal.
 32. The sensor of claim 28, whereinthe beam forming parameters include a selected angular spread of thereceived RF signal.
 33. The sensor of claim 32, wherein the selectedangular spread is from 1 to 90 degrees.
 34. The sensor of claim 33,wherein the selected angular spread is 20 degrees.
 35. The sensor ofclaim 28, wherein each of the N lasers generates radiation with awavelength, λ_(N), being different from wavelengths of any other lasers;and wherein the combiner comprises one or more wavelength divisionmultiplexing (WDM) combiners; the WDM combiner being followed by a GroupArray Photodetector Combiner (GAPC) device; the GAPC device combiningoutputs of multiple WDM combiners.
 36. The sensor of claim 28, furthercomprising N Local oscillator (LO) lasers frequency locked with the Nlasers; each LO laser producing one of a set of n LO signals enteringone of a set of n LO phase delays followed by one of a set of n LOamplitude controllers, where 1≤n≤N.
 37. The sensor of claim 36, whereinthe output signals from N modulators and N LOs are each split into Midentical copies using two optical power splitters, wherein M≥2, the Midentical copies being used to simultaneously provide M independent RFoutput signals, each RF output signal having independently chosen beamforming parameters by selecting appropriate time delays, phase delays,and attenuations.
 38. The sensor of claim 28, further comprising Nsplitters receiving the optical carriers from the N lasers thus formingN signal channels and N Local Oscillator (LO) channels; each of the Nmodulators receiving optical inputs in each signal channel; furthercomprising N LO modulators receiving optical inputs in the LO channels;each of the LO modulators modulate the optical carrier in acorresponding LO channel.
 39. The sensor of claim 38, wherein each ofthe N lasers generates radiation with a wavelength, λ_(N), beingdifferent from wavelengths of any other laser; and wherein the combinercomprises one or more wavelength division multiplexing (WDM) combiners;the WDM combiner being followed by a Group Array Photodetector Combiner(GAPC) device; the GAPC device combining outputs of multiple WDMcombiners.
 40. A phased array sensor, comprising: an array of N antennareceiving elements, where N≥2, that receive a free space radio frequency(RF) signal; the RF signal being a mixture of RF signals coming from alldirections; wherein each of the receiving elements converts the receivedRF signal into an antenna element electrical signal; a combination ofmultiple photonic integrated circuit (PICs), the combination of multiplePICs including: N optical modulators configured to receive opticalcarriers from N lasers, the N modulators modulate the optical carrierswith the antenna element electrical signal received from a correspondingone of the antenna elements; wherein all output optical signals from theN modulators pass through a series of Y tunable filters; each filter inthe series of tunable filters selects a frequency band fordown-conversion, and passes the remaining modulated signal along to anext filter in the series of tunable filters; wherein a signal from eachof the filters passes through a corresponding time delay (TD) element,each of the TD elements imparts an individual time delay; wherein the TDelements are followed by amplitude controllers that apply individualattenuation; each output of each of the amplitude controllers iscombined in one of a set of y couplers with a Local Oscillator (LO)signal from one of a set of Y local oscillators (LOs); wherein 1≤y≤Y;wherein all of the Y LOs are frequency locked with one laser from the Nlasers; wherein output signals from each of the LOs pass through a phaseand amplitude control device prior to entering the one of a set of ycouplers; wherein, via a digital signal processor, all time delays,phase delays and attenuations are selected to collectively provide fromthe free space RF signal only an RF signal with chosen beam formingparameters; wherein output optical signals from all of the y couplersare combined in a combiner comprising a GAPC, wherein the electricaloutputs from the group of photodetectors on each PIC are connected to anadjacent group on an adjacent PIC via inductors, each said GAPCoutputting a single electrical output, the amplitude and phase of whichcontain information about the RF signal with chosen beam formingparameters received by the N antenna receiving elements.